The present invention relates to radar transmitter/receivers for use for example in automotive vehicles.
Various types of distance measuring sensor are presently being considered for use in automotive safety programmes, the principle application being referred to as Autonomous Intelligent Cruise Control (AICC).
A vehicle equipped with conventional cruise control can be made to hold a road speed pre-selected by the driver until the brake pedal is operated, whereupon the vehicle reverts to normal operation. AICC enables a conventional vehicle cruise control function to be extended whereby the braking and throttle functions of cruise control operate without driver intervention. This function is achieved by the sensor on the AICC vehicle being able to determine the distance, lateral position across the carriageway and relative speed of nearby vehicles. Thus, a vehicle operating in autonomous cruise control mode can be automatically brought to a standstill if an object is detected in the path of the vehicle. The pre-selected cruise speed may be resumed once the object is no longer in the path of the AICC vehicle, giving the vehicle autonomous control in stop-go traffic.
One type of radar that is particularly suited to this application is frequency modulated continuous wave (FMCW), primarily because of its simplicity, potential low cost, and relative ease of subsequent signal processing.
In such a radar, a voltage controlled oscillator (VCO) is arranged to generate R.F. energy at nominally 76.5 GHz, which frequency may be varied over a range by applying a voltage ramp of known duration, giving a known frequency ramp rate. If this RF energy is then transmitted via an antenna to a reflective target, during the time taken for the energy to reach the target and return to the antenna the frequency of the VCO will have changed, and by mixing the reflected signal with the newly generated oscillator frequency used as the local oscillator (L.O.) drive to the mixer the distance-to-target can be calculated from the difference frequency.
In principle there may be two antennae, one for transmit and one for receive. In practice a single antenna is desirable to minimise size and cost, particularly for automotive use. In order to implement an FMCW radar in a single antenna (monostatic) form, it is normally the case that the transmit and receive functions are combined in at the antenna by means of a microwave circulator.